Feldspars

Feldspars are the most abundant group of minerals within the earth’s crust. There are many varieties of feldspar, distinguished by variations in chemistry and crystal structure. Although feldspars have some economic uses, their principal importance lies in their role as rock-forming minerals.

Composition and Properties

Considered as a group, the feldspars are the most abundant minerals in the earth’s crust. They form in igneous, metamorphic, and sedimentary rocks and are among the principal repositories for sodium, calcium, potassium, and aluminum in the crust. The feldspars are thus compounds of aluminum, oxygen, and silicon, together with one or more of the elements sodium, potassium, and calcium. They form two principal series: potassium feldspars and plagioclase feldspars. There are a few rare barium feldspars as well. The feldspars are a more complex group than this summary suggests at first glance, as they undergo subtle but important changes in crystal structure depending on temperature and pressure. Also, there are several distinctive mixtures of potassium and plagioclase feldspars.

The physical properties of all the feldspars are similar. They all have a Mohs scale hardness of 6 (they can scratch most glass, but cannot scratch quartz). Their densities are all in the range of 2.6-2.75 grams per cubic centimeter, or about the average density of most common rock-forming minerals. Feldspars are usually, though not always, light in color. They are usually translucent in thin splinters but in rare cases can be transparent. All the feldspars have good cleavage, or a tendency to split easily along smooth planes dictated by the atomic structure of the mineral. They cleave along two perpendicular or nearly perpendicular planes.

Crystal Structure

Most of the feldspars crystallize in the triclinic crystal system; a few feldspars are monoclinic. Crystals are classified according to their atomic arrangements. The fundamental atomic unit that makes up any crystalline material can be pictured as fitting inside an imaginary box, or unit cell, with parallel sides. Unit cells stack together to form a crystal, and the shape of the crystal reflects the shape of its unit cell. In triclinic crystals, the angles between the faces or edges of the unit cell are never right angles. In monoclinic crystals, two pairs of faces of the unit cell are perpendicular, but the third is not. A monoclinic feldspar unit cell looks like a carton with no top or bottom, sheared slightly out of shape so that its outline is an oblique parallelogram instead of a rectangle.

One feature of the crystal structures of the feldspars is especially notable. The feldspar minerals have a pronounced tendency to exhibit distinctive kinds of crystal twinning, or abrupt changes in crystal growth patterns. The growth of a crystal can be pictured as stacking planes of atoms on one another in a specific pattern. There are often many equally possible ways to stack one plane on the next. When a crystal has been built up according to one stacking pattern and then begins following a different pattern, there is an abrupt change in the atomic structure of the mineral. This changeover of atomic structure is called twinning. Sometimes the results are visible to the unaided eye, and the mineral appears to consist of two crystals stuck to one another or penetrating each other. In other cases, the results of twinning may only be visible through the microscope. The twinning of feldspars is a valuable clue to the geologist, because twinning often makes it easy to distinguish feldspars that are otherwise very similar.

Feldspars belong to the tectosilicate group: silicate minerals in which silica tetrahedra link to form three-dimensional networks. The silica tetrahedra in feldspars link to form zigzag chains, and the chains in turn are linked to adjacent chains to create a continuous network. The aluminum in feldspars occupies the centers of some of the tetrahedra in place of silicon, and the potassium, sodium, or calcium occupy the open spaces between the chains.

Potassium Feldspars

There are three important potassium feldspars: microcline, orthoclase, and sanidine. Microcline is perhaps the most familiar potassium feldspar, because it is the normal potassium feldspar in granitic rocks and the principal potassium feldspar in metamorphic rocks. Microcline can be any light color but is most often white or pink. The familiar pink color of many granites is caused by microcline, which is colored pink by microscopic plates of the iron oxide mineral hematite within the feldspar. Amazonite is a distinctive variety of microcline, unusual for its bright green color. Amazonite is a minor gemstone. The green color is of uncertain origin. It has been attributed to small amounts of the elements rubidium or lead, or to changes in the crystal structure of the microcline because of natural radioactivity in the surrounding rocks. Colors in minerals frequently have complex causes and are often the result of very tiny amounts of impurities. It is common for a given color to have several different causes. Orthoclase is a variety of potassium feldspar that forms at somewhat lower pressure than does microcline. It occurs chiefly in granitic rocks that form near the earth’s surface and cool quickly, and also in volcanic rocks. Sanidine is a high-temperature potassium feldspar found in some volcanic rocks. All three varieties of potassium feldspar are distinguished by differences in their crystal structure, particularly as seen under the microscope.

Plagioclase Feldspars

The plagioclase series of feldspars is one of the best natural illustrations of a solid solution. Just as solutions exist in liquids, a solid solution is a blend of two or more distinct materials on the atomic scale. Metallic alloys are other familiar examples of solid solutions. Solid solutions differ from chemical compounds in that the components can have variable proportions. They differ from simple mixtures (for example, salt and pepper) in that the components are interchangeable on the atomic level. These concepts are important to understand because feldspars include both solid solutions and mixtures.

The plagioclase series consists of a solid solution of two components, or end members: albite and anorthite. The proportions of aluminum and silicon are different in anorthite because calcium ions in minerals normally have a +2 electric charge, compared with the +1 of sodium. Therefore, an aluminum ion (+3) must substitute for silicon (+4) to compensate for the extra charge on the calcium ion. The plagioclase series is subdivided into six members, depending on the relative amounts of albite and anorthite. In increasing order of anorthite content, the plagioclase feldspars are albite, oligoclase, andesine, labradorite, bytownite, and anorthite. The plagioclase feldspars become somewhat denser and usually darker in color with increasing anorthite content.

Albite contains less than 10 percent of the anorthite component and forms in sodium-rich environments. Albite forms in marine sedimentary rocks as a cementing mineral, and forms in marine volcanic rocks when sodium from seawater replaces calcium in their plagioclase feldspars. Albite can also form during the metamorphism of sodium-rich rocks. Albite is rare in igneous rocks because igneous rocks are rarely so rich in sodium and poor in calcium that the mineral would form. Oligoclase contains 10 to 30 percent of the anorthite component and is very common because it is the normal plagioclase feldspar in granitic rocks. Andesine contains 30 to 50 percent anorthite and is common in igneous rocks that are less silica-rich than granite, such as diorite. Labradorite contains 50 to 70 percent anorthite and is the principal plagioclase feldspar in silica-poor igneous rocks such as basalt or gabbro. Labradorite is also the principal feldspar in rare igneous rocks called anorthosite, which consist mostly of plagioclase. Anorthosite is a common rock type of the moon. Terrestrial anorthosites, rare as they are, are commonly used as ornamental building stones. They generally are dark gray with large feldspar crystals a centimeter or more across, and show attractive bright bluish reflections from cleavage planes within the feldspar. Bytownite, with 70 to 90 percent anorthite, is perhaps the rarest plagioclase. It occurs most often in very silica-poor igneous rocks. Anorthite is any plagioclase with 90 percent or more of the anorthite end member (or less than 10 percent albite). Anorthite most often forms through the metamorphism of rocks rich in calcium and aluminum but very poor in sodium, such as clay-rich limestones.

Other Feldspars

To some extent, potassium feldspars and sodium-rich plagioclases also form a solid solution. Feldspars containing roughly equal parts potassium feldspar and albite are called anorthoclase. In addition to pure feldspar minerals, there exist a wide variety of mixtures of feldspars. At high temperatures, potassium feldspars and plagioclases coexist in solid solution much more readily than at low temperatures. Crystals that formed a stable solid solution when an igneous or metamorphic rock formed often become unstable when the rock cools. As the feldspar cools, plagioclase and potassium feldspar often separate. The final result is a patched or streaked network of plagioclase and potassium feldspar filaments interlocked with one another. This texture is easily visible to the unaided eye. When the feldspar consists mostly of potassium feldspar enclosing small amounts of plagioclase, the mixture is called perthite. Perthite is very common; almost any large microcline crystal will exhibit perthitic texture, with the pink microcline enclosing milky filaments of plagioclase. Less often, plagioclase is the dominant feldspar, enclosing small inclusions of potassium feldspar. Such a mixture is called antiperthite.

There are very few feldspar minerals other than the potassium and plagioclase feldspars. The openings in the atomic structure of feldspar are so large that only very large ions can be held there. Magnesium and iron ions are too small, so there are no magnesium or iron feldspars. Lithium, although chemically similar to sodium and potassium, is also too small to form feldspars. Cesium and rubidium feldspars have been made artificially; some feldspars are rich in cesium and rubidium, but no special names have been assigned to these minerals. Barium feldspars, such as celsian, do exist in nature. Hyalophane can be considered a solid solution of celsian, albite, and potassium feldspar. Banalsite is another barium feldspar. The rare mineral buddingtonite forms when ammonia-rich volcanic solutions alter plagioclase, replacing sodium and calcium with ammonia.

Feldspar-like Minerals

A few feldspars and feldspar-like minerals form when other small ions substitute for aluminum or silicon within the silica tetrahedra. Feldspar-like minerals that form in this manner include reedmerg-nerite, eudidymite, danburite, and hurlbutite. All are uncommon.

A few minerals that are geologically akin to the feldspars deserve mention. The feldspathoids have feldspar-like chemical compositions and form when rocks are too poor in silica to form feldspars. They are usually softer than feldspars and with quite different crystal forms. The scapolite minerals are essentially plagioclase feldspars that include molecules of sodium chloride (halite or table salt), calcium sulfate (gypsum), or calcium carbonate (calcite) within their atomic structures. It has been suggested that scapolite may be a common mineral on Mars, formed when plagioclase absorbed carbon dioxide from the planet’s atmosphere, and that much of the planet’s original carbon dioxide may now be locked up in scapolite.

Techniques for Identifying Feldspars

A wide range of techniques has been developed to probe the structure of minerals with polarized light. The two most obvious features of minerals in polarized light are interference color and extinction. When light enters a mineral, it splits into two beams polarized in perpendicular planes. The orientation of the planes is closely related to the crystal structure of the mineral. The two light beams travel at different speeds through the mineral. When the beams emerge, they recombine into a single light beam whose direction of vibration is usually different from the original direction. Some light passes through the second polarizer. The resulting interference color is determined strictly by the amount the two beams of light separated within the mineral. If the specimen is rotated (petrographic microscopes are normally equipped with rotating specimen stages), it will black out, or undergo extinction, at intervals of 90 degrees. Extinction occurs when the vibration directions of light in the mineral match those of the polarizing filters. In these positions, light leaving the mineral experiences no change in vibration direction and is blocked by the second polarizing filter.

Under the microscope, in normal illumination, feldspars are colorless and similar in appearance to quartz. They can often be distinguished from quartz by cleavage (which appears as straight, parallel cracks) and by a dusty appearance caused by chemical alteration. Quartz is almost immune to chemical alteration. Between crossed polarizers, quartz and feldspar display similar interference colors, but the twinning habits of the feldspars usually make it easy to distinguish them. Because the crystal structure changes abruptly when twinning occurs, twinned crystals are obvious as a result of abrupt changes in the optical properties. A crystal that looks like a single entity in normal illumination appears as distinct regions with different interference color or extinction between crossed polarizers.

Geologists sometimes use staining techniques to identify feldspars. The most common method involves etching the specimen with hydrofluoric acid (an extremely hazardous material) and applying a series of chemicals that stain potassium feldspars yellow and plagioclase pink.

For probing the atomic structure of feldspars in detail, many highly sophisticated techniques are in use. The chemical composition of feldspars can be determined for even tiny specimens by bombarding the specimen with electrons (electron microprobe) or with X-rays (X-ray fluorescence) and measuring the energies of radiation given off by the specimen. The atomic arrangement of feldspars is determined by X-ray diffraction, in which X-rays are reflected off atoms within the specimen. The number of X-rays reflected in different directions can be used to determine the geometric arrangement of atoms within the crystal.

Geologic Significance

Feldspars have some economic value, but their principal importance lies in their role as major building blocks of the earth. Because they are such tremendously important reservoirs of common elements, feldspars are key minerals in classifying rocks, and the composition of feldspar minerals in a rock is a powerful clue to its origin and history. Feldspars are also geologically significant for their role in radiometric dating. Both the potassium-argon and rubidium-strontium dating methods rely on elements that are found in feldspar, either as principal ingredients (potassium) or as common trace elements (rubidium and strontium). Because feldspars are significant, researchers continue to study them. In 2024, they discovered how feldspars in the atmosphere contribute to the formation of clouds.

Economic Value

Feldspars have minor use as gemstones. Amazonite is a green variety sometimes used as a gem, and moonstone is translucent feldspar with microscopic inclusions that give it a milky appearance. Aventurine is a clear feldspar with tiny plates of other minerals that impart it with a sparkly appearance. Some feldspar-rich rocks called anorthosite are used as an ornamental building stone.

The most important uses of feldspar are less glamorous. Feldspar is the principal ingredient of porcelain. Indirectly, feldspar is the source of aluminum. Rocks that are rich in feldspar and poor in iron are broken down by tropical weathering so that all but the aluminum is dissolved away. The final result is a mixture of aluminum minerals called bauxite, the principal ore of aluminum. In temperate climates, weathering of feldspar releases potassium, an essential plant nutrient.

Principal Terms

crystal: a material with a regular, repeating atomic structure

igneous rocks: rocks formed from the molten state; they may be erupted on the surface (volcanic) or harden within the earth’s crust (plutonic or intrusive)

ion: an atom that has gained or lost electrons and thereby acquired an electric charge; most atoms in minerals are ions

metamorphic rocks: rocks formed by the effects of heat, pressure, or chemical reactions on other rocks

polarized light: light whose waves vibrate or oscillate in a single plane

sedimentary rocks: rocks formed on the earth’s surface from materials derived by the breakdown of previously existing rocks

silicate: a mineral containing silica tetrahedra, which may be separate from one another or joined into larger units by sharing their corner oxygen atoms

silica tetrahedron: the fundamental molecular unit of silica; a silicon atom bonded to four adjacent oxygen atoms in a three-sided pyramid arrangement

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